Event localization and fall-off correction by distance-dependent weighting

ABSTRACT

A nuclear camera system includes a detector ( 12 ) for receiving radiation from a subject ( 14 ) in an exam region ( 16 ). The detector ( 12 ) includes a scintillation crystal ( 20 ) that converts radiation events into flashes of light. An array of sensors ( 22 ) is arranged to receive the light flashes from the scintillation crystal ( 20 ). Each of the photomultiplier sensors ( 22 ) generates a respective sensor output value in response to each received light flash. A processor ( 26 ) determines when each of the radiation events is detected. At least one of an initial position and an energy of each of the detected radiation events is determined in accordance with respective distances (d 1  . . . d 19 ) from a position of the detected event to the sensors ( 22 ). An image representation is generated from the initial positions and energies.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/209,032, filed Jun. 2, 2000.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to the art of nuclear medicine anddiagnostic imaging. It finds particular application in localizing ascintillation event in a gamma camera having a number ofphotomultipliers arranged over a camera surface. It is to be appreciatedthat the present invention may be used in conjunction with positronemission tomography (“PET”), single photon emission computed tomography(“SPECT”), whole body nuclear scans, transmission imaging, otherdiagnostic modes and/or other like applications. Those skilled in theart will also appreciate applicability of the present invention to otherapplications where a plurality of pulses tend to overlap, or “pile-up”and obscure one another.

[0003] Diagnostic nuclear imaging is used to study a radio nuclidedistribution in a subject. Typically, one or more radiopharmaceutical orradioisotopes are injected into a subject. The radiopharmaceutical arecommonly injected into the subject's bloodstream for imaging thecirculatory system or for imaging specific organs that absorb theinjected radiopharmaceutical. A gamma or scintillation camera detectorhead is placed adjacent to a surface of the subject to monitor andrecord emitted radiation. Each detector typically includes an array ofphotomultiplier tubes facing a large scintillation crystal. Eachreceived radiation event generates a corresponding flash of light(scintillation) that is seen by the closest photomultiplier tubes. Eachphotomultiplier tube that sees an event generates a corresponding analogpulse. Respective amplitudes of the pulses are generally proportional tothe distance of each tube from the flash.

[0004] A fundamental function of a scintillation camera is eventestimation, which is the determination of energy and position of thelocation of an interacting gamma or other radiation ray based on thedetected electronic signals. A conventional method for event positioningis known as the Anger method, which sums and weights signals seen bytubes after the occurrence of an event. The Anger method for eventpositioning is based on a simple first moment calculation. Morespecifically, the energy is typically measured as the sum of all thephotomultiplier tube signals, and the position is typically measured asthe “center of mass” of the photomultiplier tube signals.

[0005] Several methods have been used for implementing the center ofmass calculation. With fully analog cameras, all such calculations(e.g., summing, weighting, dividing) are done using analog circuits.With hybrid analog/digital cameras, the summing and weighting are doneusing analog circuits, but the summed values are digitized and the finalcalculation of position is done digitally. With “fully digital” cameras,the tube signals will be digitized individually. In any event, becausethe fall-off curve of the photomultipliers is not linear as assumed bythe Anger method, the image created has non-linearity errors.

[0006] One important consideration is the location of the eventestimation. The scintillation light pulse is mostly contained within asmall subset of the tubes on a detector. For example, over 90% of atotal signal is typically detected in seven (7) out of a total number oftubes, typically on the order of 50 or 60. However, imaging based onlyon the seven (7) closest tubes, known as clustering, has poor resolutionand causes uniformity artifacts. Furthermore, because thephotomultiplier tubes have non-linear outputs, the scintillation eventsare artificially shifted toward the center of the nearestphotomultiplier tube.

[0007] For a given detector geometry, the fall-off curve varies with adepth that a gamma photon interacts in the crystal. Different energyphotons have varying interaction depth probabilities that are morepronounced in thicker crystals, which are typically used in combinationwith PET/SPECT cameras.

[0008] Therefore, separate linearity or flood correction tables arecreated and used for each energy in order to correct for the uniformityartifact. Fall-off curves are acquired using a labor intensive method ofmoving a point source a small amount (e.g., 2 mm) roughly 30-40 timesfor each tube. The individual tube's output is acquired at eachlocation, the mean value of the tube's output is found, and a curve oftube output versus distance from the location of the point source isgenerated.

[0009] A disadvantage of generating a fall-off curve using a pointsource is the large amount of time required to move the source position.This method is also prone to errors in positioning the source accuratelyon the detector. It is also usually only done in one or two directionsTherefore, the assumption is made that the fall-off curve is exactlysymmetric. Regenerating the fall-off curve for a different energyrequires that the process be repeated again. Likewise, generating thefall-off curve for a different tube requires the process be repeatedagain. Therefore, the assumption is usually made that the fall-off curveis invariant across different detectors or photomultiplier tubes.

[0010] Generating the linearity correction tables typically involvesusing a lead mask that contains many small holes to restrict theincident location of radiation on the crystal surface. The holesrepresent the true location of the incident photons that interact in thedetector crystal. This information is used to generate a table thatconsists of x and y deltas that when added to the x and y estimate,respectively, are used to generate a corrected position estimate thatmore accurately reflects the true position. A disadvantage is that newtables must be generated for each energy that is to be imaged, therebyincreasing the calibration time. Another disadvantage is that thecalibration mask has a limited number of holes, since each must beresolved individually, thereby limiting the accuracy of the correction.It is also increasingly more expensive and difficult to calibrate forhigher energy photons since the thickness of the lead mask must increasein order to have sufficient absorption in non-hole areas.

[0011] Another prior art method uses separate flood uniformitycorrection tables for each energy. A disadvantage is that new tablesmust be generated for each energy that is to be imaged, which increasescalibration time. Flood correction has the disadvantage of creatingnoise in the image, since the method is based on either adding orremoving counts unevenly throughout the pixel matrix. This method isalso sensitive to drift in either the photomultiplier tubes orelectronics.

[0012] Another prior art method reduces the output from the closesttube. For example, an opaque dot is sometimes painted over the center ofeach photomultiplier tube. The sensitivity can also be reducedelectronically. Unfortunately, the closest photomultiplier tubetypically has the best noise statistics. Reducing its sensitivity to theevent causes a resolution loss.

[0013] Similarly, excluding the outlying tubes reduces the noise in thedetermined values of energy and position. The most common way ofexcluding signals from outlying tubes includes imposing a threshold,such that tube signals below a set value are either ignored in thecalculation or are adjusted by a threshold value. This method worksreasonably well in excluding excess noise. However, the method fails ifstray signals exist above the threshold value. Stray signals may existat high-counting rates, when events occur nearly simultaneously in thecrystal. When two events occur substantially simultaneously, their“center-of-mass” is midway between the two—where no event actuallyoccurred. Nearly simultaneously occurring events may result inpulse-pile-up in the energy spectrum and mispositioning of events. Thisbehavior is especially detrimental in coincidence imaging, wherehigh-count rates are necessary.

[0014] Thus, it is desirable to improve localization in eventestimation. With a fully digital detector, both the intensity and thelocation of each tube signal are known. It is, therefore, possible tocalculate the energy and position based primarily on the tube signalsclose to an individual event. One current method for event localizationis seven (7) tube clustering in which a cluster of seven (7) tubes isselected for each event. These tubes include the tube with maximumamplitude, along with that tube's six (6) closest neighbors. This methodis an effective method for limiting the spatial extent of thecalculation. However, the main drawback of this method is the resultingdiscontinuity.

[0015] Discontinuity arises when the detected positions for events froma uniform flood source form an array of zones around each possiblecluster. Elaborate correction schemes (see e.g., Geagan, Chase, andMuehllehner, Nucl. Instr. Meth. Phys. Res A 353, 379-383 (1994)) areneeded to “stitch” together these overlapping zones to form a single,continuous image. However, this correction is sensitive to electronicshifts, which often arise in high-count situations, causing seamartifacts in the camera response.

[0016] The present invention provides a new and improved apparatus andmethod which overcomes the above-referenced problems and others.

SUMMARY OF THE INVENTION

[0017] A nuclear camera system includes a detector for receivingradiation from a subject in an exam region. The detector head includes ascintillation crystal, which converts radiation events into flashes oflight, and an array of sensors, which are arranged to receive the lightflashes from the scintillation crystal. Each of the sensors generates arespective sensor output value in response to each received light flash.A processor determines when each of the radiation events is detected. Atleast one of an initial digital position and an energy of each of thedetected radiation events is determined in accordance with respectivedistances from a position of the detected event to the sensors. An imagerepresentation is generated from the digital positions.

[0018] In accordance with one aspect of the invention, each of thesensors is electrically connected to at least one of a plurality ofanalog-to-digital converters for converting the sensor output valuesfrom analog values to respective series of digital sensor output values.

[0019] In accordance with another aspect of the invention, the processorweights the sensor output values with weighting values for determiningcorrected positions of the events. The weighting values are determinedin accordance with the respective distances from the position of eachevent to each of the sensors that detects the event.

[0020] In accordance with a more limited aspect of the invention, theprocessor determines a subsequent set of weighting values as a functionof the corrected positions and energies of the events.

[0021] In accordance with another aspect of the invention, the processorgenerates the weighting values for each of the distances as a functionof a desired response curve and an input response curve.

[0022] In accordance with a more limited aspect of the invention, theprocessor generates the weighting values as a function of the energybeing imaged.

[0023] In accordance with an even more limited aspect of the invention,the processor generates energy ratio curves representing respectiverelationships between a plurality of the energies being imaged. Theprocessor generates an energy scaling curve representing a relationshipbetween the plurality of energies being imaged and respective scalingfactors. Also, the processor generates the weighting values as afunction of one of the scaling factors.

[0024] In accordance with another aspect of the invention, a look-uptable is accessed by the processor for storing the weighting values.

[0025] In accordance with a more limited aspect of the invention, thelook-up table is multi-dimensional and indexed as a function of at leastone of time, temperature, count-rate, depth of interaction, and eventenergy.

[0026] In accordance with another aspect of the invention, the processoranalyzes the sensor output values for detecting a start of the event.

[0027] In accordance with a more limited aspect of the invention, theprocessor analyzes the sensor output values for detecting a previousevent. Any sensor output values associated with the previous event areexcluded from calculations of an initial position and an energy of anext detected event.

[0028] In accordance with another aspect of the invention, in responseto the processor detecting a next event after an integration period ofthe event begins, during which the position of the detection event isdetermined, the sensor values associated with the sensors of the nextevent are nulled from calculations of the initial position and theenergy of the event.

[0029] In accordance with another aspect of the invention, a seconddetector disposed across an imaging region from the first detector. Acoincidence detector is connected with the first and second detectorsfor detecting concurrent events on both detectors. A reconstructionprocessor determines rays through the imaging region between concurrentevents and reconstructs the rays into an output image representation.

[0030] In accordance with another aspect of the invention, an angularposition detector determines an angular position of the detector aroundan imaging region. A reconstruction processor is connected with thedetector and the angular position detector for reconstructing avolumetric image representation from the corrected positions of theevents on the detector and the angular position of the detector duringeach event.

[0031] In accordance with another aspect of the invention, the sensorsinclude photomultiplier tubes.

[0032] One advantage of the present invention resides in its highlinearity. Therefore, linearity and uniformity corrections are reduced.

[0033] Another advantage resides in improved accuracy in eventpositioning, even in high count and pile-up situations.

[0034] Another advantage is that local centroiding is continuous andseamless.

[0035] Another advantage resides in more accurate estimation of events.

[0036] Still further advantages of the present invention will becomeapparent to those of ordinary skill in the art upon reading andunderstanding the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention may take form in various components andarrangements of components, and in various steps and arrangements ofsteps. The drawings are only for purposes of illustrating a preferredembodiment and are not to be construed as limiting the invention.

[0038]FIG. 1 is a diagrammatic illustration of a nuclear camera systemaccording to the present invention;

[0039]FIG. 2 illustrates an overview flowchart according to the presentinvention;

[0040]FIG. 3 illustrates a flow chart detailing the flowchart shown inFIG. 2;

[0041]FIG. 4 illustrates a partial array of sensors;

[0042]FIG. 5 illustrates a graphical depiction of an event in amplitudeversus time;

[0043]FIG. 6 illustrates an optimal weighting graph according to thepresent invention in multiplier correction value versus distance;

[0044]FIG. 7 illustrates an actual fall-off curve used for obtaining theoptimal weighting graph of FIG. 6;

[0045]FIG. 8 illustrates a desired fall-off curve used for obtaining theoptimal weighting graph of FIG. 6;

[0046]FIG. 9 illustrates a flowchart for generating a scaling curveaccording to the present invention;

[0047]FIG. 10 illustrates various energy ratio curves according to thepresent invention;

[0048]FIG. 11 illustrates an energy scaling curve according to thepresent invention;

[0049]FIG. 12 illustrates a flow chart detailing the flowchart shown inFIG. 3; and

[0050]FIG. 13 illustrates an embodiment of the present inventionincluding a PET scanner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] With reference to FIG. 1, a nuclear camera system 10 includes aplurality of detectors heads (“detectors”) 12 mounted for movementaround a subject 14 in an examination region 16. Each of the detectors12 includes a scintillation crystal 20 that converts a radiation eventinto a flash of light energy or scintillation. An array of sensors 22,e.g. 59 sensors, is arranged to receive the light flashes from thescintillation crystal. In the preferred embodiment, the sensors includephotomultiplier tubes. However, other sensors are also contemplated.

[0052] Each of the sensors 22 generates a respective analog sensoroutput pulse (e.g., tube output pulse) in response to the received lightflash. Furthermore, each of the sensors 22 is electrically connected toanalog-to-digital converters 24. The analog-to-digital converters 24convert the analog sensor output pulses to a series of digital sensoroutput values, as illustrated in FIG. 5. As is discussed in more detailbelow, a processor 26 determines coordinates in two dimensions of thelocation and the energy of the scintillation event that occurred in thecrystal.

[0053] With reference to FIGS. 1 and 2, radiation is detected andconverted into sensor output values (e.g., tube output values), whichare transmitted to the processor 26 in a step A. Then, in a step B, theprocessor 26 detects that an event occurs and identifies which sensorvalues (e.g., tube values) will be used for determining an approximateposition and energy of the event. In a step C, the processor 26calculates the approximate position and energy of the event and thendetermines a corrected position by applying a weighting algorithm.Finally, in a step D, an image (e.g., volumetric image) isreconstructed.

[0054] With reference to FIGS. 2 and 3, each of the steps A-C includes aplurality of respective sub-steps, which are discussed below. For easeof explanation, each of the sub-steps is identified with a referencenumeral specifying both the step (see FIG. 2) and the sub-step (see FIG.3).

[0055] With reference to FIGS. 1-3, each radiation event is detectedwithin the array of sensors 22 in a sub-step A1. The radiation producesgamma quanta that arise in the disintegration of radioisotopes. Thedisintegration quanta strike the scintillation crystal, which preferablyincludes doped sodium iodide (NaI) causing a scintillation. Light fromthe scintillation is distributed over a large number of the sensors 22.

[0056] As illustrated in FIG. 4, the scintillation, which is created bya radiation event, is illustrated centered at an arbitrary position 28.It is to be understood that only a partial array of the sensors 22 isshown in FIG. 4.

[0057] With reference to FIGS. 1, 3, and 4, the energy of the absorbedgamma quantum is converted, or transformed, into the flash of light atthe position 28 by the scintillation crystal in a sub-step A2. Thesensors 22 detect (receive) the scintillation light in a sub-step A3.Then, the sensors 22 produce the respective analog sensor output signalsin a sub-step A4. The relative strengths of the analog sensor outputsignals are proportional to the respective amounts of the scintillationlight received by the sensors 22 in the sub-step A3. Theanalog-to-digital converters 24 convert the analog sensor output signalsto respective series of digital sensor output values in a sub-step A5.The digital sensor output values are then transmitted to the processor26 in a sub-step A6.

[0058] Referring now to FIGS. 1 and 3-5, a scintillation event 28typically includes a rapidly changing portion 40, which reaches a peak42. The processor 26 detects that an event occurs (starts) in a sub-stepB1 by analyzing the output values for each of the sensors. In thepreferred embodiment, the processor 26 triggers (detects) that an eventoccurs when a sensor output value surpasses a trigger amplitude 44.

[0059] For the processor to determine the energy of the event 28, thearea underneath the curve is determined. The signal is sampled at a ratesufficient to capture an appropriate number of amplitude values. A ratebetween 40 to 70 MHz provides a useful number of samples. Artisansappreciate with further reference to FIG. 5, that the integration orcombination of sample data points is relatively straight-forward for asingle scintillation event. The integration becomes problematic whenseveral pulses overlap, a condition known as pile-up.

[0060] As discussed above, a post-pulse pile-up occurs when a subsequentevent is detected during an integration period of the first event. Apre-pulse pile-up occurs when the processor 26 indicates the presence ofa previous event that occurred before the current event that is beingintegrated. The processor 26 checks for a pre-pulse pile-up in asub-step B2. In particular, the processor 26 checks whether the sensoroutputs exceed a predetermined nominal or baseline value, which wouldexist in the absence of light. To avoid the undesirable effects of pulsepile-up, the integrated values of these sensors are zeroed (nulled).

[0061] The sensor output values are integrated, during an integrationperiod, for each sensor in a sub-step B3. Subsequent triggers aredetected after a delay period (post-pulse pile-up) (e.g., 75nanoseconds), which begins substantially simultaneously when theintegration period begins, in a sub-step B4. The integration valuesassociated with the subsequent, post-pulse pile-up triggers are zeroedin a sub-step B5. It is assumed that all of the sensors 22 in theimmediate vicinity of the first event 28 have already caused the triggerprocessor 26 to trigger within this delay period for the first event 28.If the baseline processor indicates the presence of a previous event(pre-pulse pile-up), the integrated value of the corresponding sensor isalso zeroed (nulled).

[0062] It is noted that the subsequent scintillation events willintroduce some error. More specifically, the sensors which see thesubsequent scintillation events sufficiently strongly to reach thetriggering threshold are zeroed (nulled). However, the peripheralsensors that only saw a small fraction of the light from the subsequentscintillation events still have their outputs incorporated into thesummation, which determines the position or the energy of the firstscintillation event 28. It is assumed, however, that the outputs fromthese peripheral sensors are small enough, when compared to the totalsummation, that the error they contribute is negligible.

[0063] In a sub-step B6, a subset of nineteen (19) sensors, includingthe sensor 22 having a maximum integrated value along with a group(e.g., 18) of nearest sensors, are selected. Then, in a sub-step C1, theprocessor determines the approximate position 28′ and energy of theevent 28 using the subset of nineteen (19) sensors within the array ofsensors 22, preferably using weighted sums to determine a centroid(e.g., the Anger algorithm). Looking to the nineteen (19) sensorsclosest to the event 22 ₁, 22 ₂, 22 ₃, . . . , 22 ₁₉, it is assumed thatthe intensity of light received by each sensor is proportional to acorresponding distance d₁, d₂, d₃, . . . , d₁₉, between the sensor andthe event. This linear proportionality places the event at the point 28′in FIG. 4. If the sensor array were linear, point 28′ would be anaccurate estimate of the actual location 28 at which the event occurred.Due to inherent non-linearities, the point 28′ is typically shifted fromthe actual event 28.

[0064] Then, in a sub-step C2, the processor 26 determines weighting(correcting) values as a function of the respective distances from thepoint 28′ to the centers of the sensors 22, in the nineteen (19) sensorexample, a weighting function for each of distances d₁, d₂, . . . , d₁₉.In the preferred embodiment, the weighting values are assigned from anoptimal weighting graph 50 as shown in FIG. 6. With reference to FIGS.4-6, the graph 50 is designed by empirical measurement with sensorshaving a diameter of about 75 mm. However, it is to be understood thatanalogous graphs can be generated for sensors having other diameters. Itis expected that graphs used for sensors having other diameters willhave similar shapes to the graph 50. More specifically, the actualfall-off, i.e. amplitude of sensor output with distance from the centerof the sensor, is measured. This actual fall-off is compared with thedesired fall-off for a linear system. The deviation in the fall-offcurves results in the weighting function of FIG. 6. That is, operatingon the actual fall-off curve with the curve of FIG. 6 results in thedesired ideal fall-off curve. Preferably, the curve of FIG. 6 isdigitized and stored in a look-up table 52. Each of the distances d₁, .. . , d₁₉ is addressed to the abscissa of the graph 50 so that acorresponding weighting factor is retrieved from the ordinate.Therefore, in the nineteen (19) sensor example, nineteen (19) weightingfactors are retrieved from the ordinate. In this manner, the response ofsensors beyond the closest seven (7) are also used in the calculationand a subset including nineteen (19) sensors is selected.

[0065] With reference to FIGS. 6-8, the graph 50 is generated as afunction of an actual fall-off curve 54 (input response curve) and adesired fall-off curve 56 (desired response curve). More specifically,as will be discussed in more detail below, the graph 50 is obtained bydividing the desired response curve 56 by the input response curve 54.In other words, the weighting values are generated for each distance bydividing the desired response curve 56 by the input response curve 54 ateach distance. The desired response curve 56 has the characteristic ofsmoothly reaching a zero (0) value at a distance chosen to include theappropriate number of sensors in the centroid. The desired curve 56 alsohas the characteristic of being substantially non-discontinuous andsubstantially linear. The input response curve is measured or modeledfor a given camera geometry, which includes crystal thickness, glassthickness, sensor diameter, and any other operating conditions.

[0066] With reference again to FIGS. 1 and 3-5, in the sub-step C2 eachof the distances d₁ through d₁₉, as well as the distances of further outsensors, are used for addressing the look-up table to determinecorresponding weighting factors. In a sub-step C3, corrected sensorvalues are generated as a function of the weighting factors. It is to beunderstood that in other embodiments, the look-up table may also beindexed as a function of time, temperature, count-rate, depth ofinteraction, and/or event energy.

[0067] The processor 26 sums the weighted values in a sub-step C4 todetermine the corrected position 28 and energy. A decision is made in asub-step C5 whether to iterate (repeat) the correction process. If it isdecided to repeat the process of correcting the event position, controlis passed back to the sub-step C2 for determining subsequent weightingvalues from the look-up table based on the corrected position 28.Otherwise, control is passed to the step D for reconstructing the image.

[0068] The camera illustrated in FIG. 1 has a SPECT mode and a PET mode.In the SPECT mode, the heads have collimators which limit receipt ofradiation to preselected directions, i.e., along known rays. Thus, thedetermined location on the crystal 20 at which radiation is detected andthe angular position of the head define the ray along which eachradiation event occurred. These ray trajectories and head angularposition from an angular position resolver 60 are conveyed to areconstruction processor 62 which back projects or otherwisereconstructs the rays into a volumetric image representation in an imagememory 64.

[0069] In a PET mode, the collimators are removed. Thus, the location ofa single scintillation event does not define a ray. However, theradioisotopes used in PET scanning undergo an annihilation event inwhich two photons of radiation are emitted simultaneously indiametrically opposed directions, i.e., 180° apart. A coincidencedetector 66 detects when scintillations on two heads occursimultaneously. The locations of the two simultaneous scintillationsdefine the end points of a ray through the annihilation event. A ray ortrajectory calculator 68 calculates the corresponding ray through thesubject from each pair of simultaneously received scintillation events.The ray trajectories form the ray calculator 68 are conveyed to thereconstruction processor for reconstruction into a volumetric imagerepresentation.

[0070] A video processor 70 processes the image representation data fordisplay on a monitor 72.

[0071] The processor 26 also determines an energy of the event 28 byintegrating, or summing, the corrected sensor output values during anintegration period. The integration period preferably lasts about 250nanoseconds, although the integration period may vary in differentscintillation crystals, radiation energies, or software applications.That is, once all of the integrated sensor outputs of FIG. 5corresponding to the event are scaled by the correction curve 50, theyare summed to determine the energy of the event.

[0072] Stated in mathematical terms, the energy E of the event 28 andthe position x of the event 28 are calculated as: $\begin{matrix}{{E = {\sum\limits_{i}{w_{i}^{E}S_{i}}}},} \\{and} \\{{x = \frac{\sum\limits_{i}{w_{i}^{x}S_{i}x_{i}}}{\sum\limits_{i}{w_{i}^{x}S_{i}}}},}\end{matrix}$

[0073] where x_(i) represents respective sensor locations, S_(i)represents the respective sensor output values, w_(i) ^(E) representsenergy weighting values, and w_(i) ^(x) represents distance weightingvalues.

[0074] In one embodiment, w_(i) ^(E) and w_(i) ^(x) are a function ofthe respective distance |x_(i)−x₀| between the sensor location x_(i) andthe initial determined position x₀ 28′ of the event 28 (see FIG. 6). Asdiscussed above, the initial position x₀ is determined as a centroid ofthe event 28. Since a detector normally consists of photomultipliersensors arranged in a two-dimensional array, calculation of the distanceusually involves computing the value of the difference between thesensor location x_(i) and x₀ for each of a plurality of coordinates. Thedifferences are squared, summed, and the square root is taken to findd_(i). In order to avoid the complexities of taking the square root, atable look-up may be used. Alternatively, a two-dimensional fall-offcorrection curve table and/or two-dimensional pre-correction table canbe indexed by the absolute values of the differences between the sensorlocation x_(i) and x₀ in order to save the step of calculating thedistance directly.

[0075] As will be discussed in more detail below, the weighting valuesw_(i) ^(x) are optionally pre-corrected as a function of the energybeing imaged.

[0076] With reference to FIGS. 9-11, a representative fall-off curve forone energy level E1 is generated in a step F1. Preferably, the energylevel E1 is a low energy within a range including 75 KeV and 511 KeV(e.g., about 75 KeV). For purposes of explanation, it is to beunderstood that the curve 54 represents the actual fall-off curve forthe energy E1. A fall-off curve (not shown) for another energy E2, E3,E4 is acquired in a step F2. The fall-off curve (including, for example,the fall-off curve 54 for the energy level El) is normalized to bewithin a range including, for example, zero (0) and 100 in a step F3.The fall-off curve for one of the energies E2, E3, E4 is divided by thefall-off curve 54 for the first energy E1 in a step F4, therebygenerating one of a plurality of energy ratio curves (pre-correctioncurves) 80, 82, 84 (see FIG. 10). The energy ratio curves 80, 82, 84represent weighting that must be applied as a function of distance to asensor's output when a respective one of the energies E2, E3, E4 isbeing imaged.

[0077] A decision is made in a step F5 whether to repeat the process ofgenerating another one of the energy ratio curves 80, 82, 84. If it isdesired to repeat the process, control returns to the step F2 foracquiring the fall-off curve for another energy. Otherwise, controlpasses to a step F6. With reference to FIG. 10, the energy ratio curve80 represents E1/E2, the energy ratio curve 82 represents E1/E3, and theenergy ratio curve 84 represents E1/E4. Although only four (4) energylevels are discussed, it is to be understood that any number of energylevels may be generated. It is noted that each of the energy ratiocurves 80, 82, 84 may be made smoother by collecting more data and/orapplying commonly known regression or curve fits.

[0078] It is evident that all of the energy ratio curves 80, 82, 84generally have a same shape but are scaled differently. Since tablespace (i.e., computer memory) is usually limited due to memory size inpractical implementations and/or time constraints prohibit acquiringcurves for all continuous energies, an additional energy scaling curvemay optionally be used.

[0079] An energy scaling curve 86 is generated by determining scalingvalues between the energy ratio curve 80, which represents E1/E2 (e.g.,the highest energy) and each of the energy ratio curves 82, 84, whichrepresent E1/E3 and E1/E4, respectively. In this manner, the energyscaling curve 86, which yields an energy scaling factor as a function ofenergy, is produced in the step F6. It is to be understood that standardmethods are used for fitting a curve to the scaling values between thevarious energy ratio curves. As will be discussed in more detail below,a scaling value sv_(i) may be obtained from the energy scaling curve 86as a function of energy.

[0080] In the current example, it is assumed that the optimal weightinggraph 50 (see FIG. 6) is calibrated for the energy E1. Therefore, oncethe energy ratio curves 80, 82, 84 are created, the optimal weightinggraph 50 (see FIG. 6) may optionally be “pre-corrected” as a function ofan energy ratio curve corresponding to the energy being imaged and adistance of the sensor. More specifically, with reference to FIGS. 3 and10-12, a distance between a sensor center and the event 28 is determinedin a sub-step C2A. Then, in a sub-step C2B, an energy pre-correctionfactor pv_(i) is optionally obtained from the graph 80 as a function ofthe distance determined in the sub-step C2A. Importantly, an appropriateone of the energy ratio curves 80, 82, 84 is selected as a function ofthe energy being imaged. A scaling value sv_(i) is optionally obtainedfrom the energy scaling curve 86 in a sub-step C2C.

[0081] A fall-off correction value fcv_(i) is obtained from the optimalweighting graph 50 as a function of distance in a sub-step C2D. Theweighting factor w_(i) ^(x) is calculated in a sub-step C2E as w_(i)^(x)=sv_(i)*pv_(i)*fcv_(i). Then, a corrected sensor output value iscalculated as S_(i) ^(x)=w_(i) ^(x)*S_(i) in the sub-step C3. Theweighting factor w_(i) ^(x) and corrected sensor output value S_(i) ^(x)are used in the above equations for energy E and position x.

[0082] In the preferred embodiment, the fall-off curves for the energiesE1, E2, E3, E4 (see, e.g., the fall-off curve 54 of FIG. 6), aregenerated by flooding an open detector with a radiation source of aknown energy. For each event that interacts in the crystal of thedetector, an estimate of the event position is determined. Then, thedistance from the event to each of the sensor centers is calculated. Inorder to have a statistically significant number of counts for eachdistance, multiple events are produced. A histogram of each sensor'soutput is created as a function of distance. It is to be understood thatthe resolution of the distances may be set according to a requiredapplication (e.g., ¼ of an intrinsic resolution of a gamma camera). Thehistograms from different sensor outputs may be combined to generate acomposite histogram for the entire detector or certain areas that cannaturally be grouped together. The mean value of each histogram is thencomputed to generate the fall-off curve as a function of distance. Thecurve can be normalized by dividing each value by the maximum fall-offvalue (e.g., the value at, the distance zero (0)).

[0083]FIG. 13 illustrates a second embodiment of the present inventionincluding single photon emission computed tomography (“SPECT”) scanner.For ease of understanding this embodiment of the present invention, likecomponents are designated by like numerals with a primed (′) suffix andnew components are designated by new numerals.

[0084] With reference to FIG. 13, a SPECT scanner 100 includes three (3)detectors 12′ mounted for movement around a subject 14′ in anexamination region 16′. The subject is injected with a radioisotope.Each of the detectors 12′ includes a scintillation crystal 20′ forconverting radiation events from the injected isotope into a flash oflight energy or scintillation. Optionally, a radiation source 102produces a fan of transmission radiation of a different energy than theinjected radiation. Collimators 104 on the detectors limit and definethe patches or rays along which each detector can receive emission andtransmission radiation. The location of the scintillation and theposition of the receiving detector uniquely determine the ray.

[0085] An array of sensors 22′, e.g. 59 sensors, is arranged to receivethe light flashes from the scintillation crystal 20′. Each of thesensors 22′ generates a respective analog sensor output pulse (FIG. 5)in response to the received light flash. Furthermore, each of thesensors 22′ is electrically connected to at least one of a plurality ofanalog-to-digital converters 24′. As discussed above, theanalog-to-digital converters 24′ convert the analog sensor output pulsesto respective series of three digital sensor output values. Also, aprocessor 26′ determines the energy and the location in two dimensionsof each scintillation on the face of the detector, hence the ray alongwhich the radiation originated. Additionally, the curves of FIGS. 6, 10,and optionally 11 are digitized and stored in respective look-up tables52′.

[0086] Once the corrected position and energy are determined on adetector 12′ at which a scintillation occurred and from the respectivepositions of the detectors, a processor 60′ reconstructs an imagerepresentation from the emission-data. When a radiation source 102 isused, the transmission data is used to correct the emission data for animproved image. The image representation is stored in an image memory62′. A video processor 70′ processes the image representation data fordisplay on a monitor 72′.

[0087] Again, the three heads can be used without collimators in a PETmode. The heads are positioned to provide uniform coverage of the regionof interest during annihilation events. A coincidence detector 66′determines concurrent events and a ray calculator 68′ calculates thetrajectory between each pair of coincident events.

[0088] The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

Having thus described the preferred embodiment, the invention is nowclaimed to be:
 1. A nuclear camera system comprising: a detector forreceiving radiation from a subject in an exam region, the detectorincluding: a scintillation crystal that converts radiation events intoflashes of light; an array of sensors arranged to receive the lightflashes from the scintillation crystal, a plurality of the sensorsgenerating a respective sensor output value in response to each receivedlight flash; and a processor for determining when each of the radiationevents is detected, at least one of an initial position and an energy ofeach of the detected radiation events being determined in accordancewith respective distances from a position of the detected event to thesensors, and generating an image representation from the initialpositions and the energies.
 2. The nuclear camera system as set forth inclaim 1, further including: a plurality of analog-to-digital converters,each of the sensors being electrically connected to at least one of theanalog-to-digital converters for converting the sensor output valuesfrom analog values to respective series of digital sensor output values.3. The nuclear camera system as set forth in claim 1, wherein theprocessor weights the sensor output values with weighting values, whichare determined in accordance with the respective distances from theposition of each event to each of the sensors that detects the event,for determining corrected positions and energies of the events.
 4. Thenuclear camera system as set forth in claim 3, wherein the processordetermines a subsequent set of weighting values as a function of thecorrected positions and energies of the events.
 5. The nuclear camerasystem as set forth in claim 3, wherein the processor generates theweighting values for each of the distances as a function of a desiredresponse curve and an input response curve.
 6. The nuclear camera systemas set forth in claim 5, wherein the processor generates the weightingvalues as a function of an energy being imaged.
 7. The nuclear camerasystem as set forth in claim 6, wherein: the processor generates energyratio curves representing respective relationships between a pluralityof the energies being imaged; the processor generates an energy scalingcurve representing a relationship between the plurality of energiesbeing imaged and respective scaling factors; and the processor generatesthe weighting values as a function of one of the scaling factors.
 8. Thenuclear camera system as set forth in claim 3, further including: alook-up table, accessed by the processor, for storing the weightingvalues.
 9. The nuclear camera system as set forth in claim 8, whereinthe look-up table is multi-dimensional and indexed as a function of atleast one of time, temperature, count-rate, depth of interaction, andenergy.
 10. The nuclear camera system as set forth in claim 1, whereinthe processor analyzes the sensor output values for detecting a start ofthe event.
 11. The nuclear camera system as set forth in claim 10,wherein the processor analyzes the sensor output values for detecting aprevious event, any sensor output values associated with the previousevent being excluded from calculations of an initial position and anenergy of a next detected event.
 12. The nuclear camera system as setforth in claim 10, wherein in response to the processor detecting a nextevent after an integration period of the event begins, the sensor valuesassociated with the sensors of the next event being nulled fromcalculations of the initial position and the energy of the event. 13.The nuclear camera system as set forth in claim 1, further including: asecond detector disposed across an imaging region from the firstdetector; a coincidence detector connected with the first and seconddetectors for detecting concurrent events on both detectors; and areconstruction processor for determining rays through the imaging regionbetween concurrent events and reconstructing the rays into an outputimage representation.
 14. The nuclear camera system as set forth inclaim 1, further including: an angular position detector for determiningan angular position of the detector around an imaging region; areconstruction processor connected with the detector and the angularposition detector for reconstructing a volumetric image representationfrom the corrected positions of the events on the detector and theangular position of the detector during each event.
 15. The nuclearcamera system as set forth in claim 1, wherein the sensors includephotomultiplier tubes.
 16. A nuclear camera system comprising: adetector for receiving radiation from a subject in an exam region, thedetector including: a scintillation crystal that converts radiationevents into flashes of light; an array of sensors arranged to receivethe light flashes from the scintillation crystal, a plurality of thesensors generating a respective sensor output value in response to eachreceived light flash; and a processor which (i) detects overlappingevents that are sufficiently temporally close that their light flashesare at least partially concurrent, (ii) determines at least one ofposition and energy of at least one of the overlapping events whilecompensating for the partially concurrent light flash of the other, and(iii) generates an image representation from the initial positions andthe energies.
 17. The nuclear camera system as set forth in claim 16,wherein the processor analyzes the sensor output values for detecting astart of each detected event.
 18. The nuclear camera system as set forthin claim 17, wherein the processor analyzes the sensor output values fordetecting an ongoing previous event and excludes any sensor outputvalues associated with the previous event from calculations of aninitial position and an energy of a detected event.
 19. The nuclearcamera system as set forth in claim 17, wherein in response to theprocessor detecting another event after an integration period of oneevent begins, the sensor values associated with the sensors of theanother event are nulled from calculations of the initial position andthe energy of the one event.
 20. The nuclear camera system as set forthin claim 16, further including: a second detector disposed across animaging region from the first detector; a coincidence detector connectedwith the first and second detectors for detecting concurrent events onboth detectors; and a reconstruction processor for determining raysthrough the imaging region between concurrent events and reconstructingthe rays into an output image representation.
 21. A method of generatingan image representation from detected radiation events, the methodcomprising: converting radiation from a subject in an examination regioninto flashes of light; receiving the flashes of light with an array ofsensors; generating respective sensor output values in response to eachreceived light flash; determining for each flash of light (i) at leastone of an initial position and an energy and (ii) distances from thedetermined initial position to each sensor which received the flash oflight; correcting each initial position in accordance with thedetermined distances; and generating an image representation from thecorrected positions.
 22. The method of generating an imagerepresentation as set forth in claim 21, further including: weightingeach of the sensor output values in accordance with the correspondingdetermined distance; and determining the corrected position and acorrected energy in conjunction with the weighted sensor output values.23. The method of generating an image representation as set forth inclaim 22, further including: iterating the steps of weighting anddetermining the corrected position and the corrected energy.
 24. Themethod of generating an image representation as set forth in claim 22,further including: generating weighting values for each of the distancesas a function of a selected response curve and an input response curve.25. The method of generating an image representation as set forth inclaim 24, further including: generating the weighting values as afunction of the energy of the radiation.
 26. The method of generating animage representation as set forth in claim 25, further including:generating energy ratio curves representing respective relationshipsbetween a plurality of radiation energies; generating an energy scalingcurve representing a relationship between the plurality of energies anda plurality of respective scaling factors; and generating the weightingvalues as a function of the scaling factors.
 27. The method ofgenerating an image representation as set forth in claim 22, furtherincluding: accessing weighting values from a look-up table.
 28. Themethod of generating an image representation as set forth in claim 27,further including: indexing the look-up table as a function of at leastone of time, temperature, count-rate, depth of interaction, andradiation energy.
 29. The method of generating an image representationas set-forth in claim 21, further including: analyzing the sensor outputvalues to detect a start of the each flash of light.
 30. The method ofgenerating an image representation as set forth in claim 29, furtherincluding: analyzing the sensor output values for detecting a previousflash; and in the step of determining at least one of the initialposition and the energy, ignoring any of the sensor output valuesassociated with the previous flash.
 31. The method of generating animage representation as set forth in claim 29, further including: inresponse to detecting a subsequent flash after an integration period ofone of the light flashes begins, ignoring the sensor values associatedwith the sensors receiving the subsequent flash when calculating theinitial position and the energy of the light flash.
 32. A method ofgenerating an image representation from detected radiation events, themethod comprising: converting radiation from a subject in an examinationregion into flashes of light; receiving the flashes of light with anarray of sensors; generating respective sensor output values in responseto each received light flash; detecting temporally adjacent lightflashes that are at least partially overlapping; determining a positionfor each non-overlapping flash of light; in each pair of overlappinglight flashes, compensating for one of the light flashes whiledetermining a position of the other; and, generating an imagerepresentation from the determined positions.
 33. The method ofgenerating an image representation as set forth in claim 22 furtherincluding: detecting a start of the each flash of light.
 34. The methodof generating an image representation as set forth in claim 33, furtherincluding: in the step of determining the position of overlapping lightflashes, ignoring any of the sensor output values associated with afirst flash while determining the position of a second flash.
 35. Themethod of generating an image representation as set forth in claim 33,further including: in response to detecting a subsequent flash after anintegration period of one light flash begins, ignoring the sensor valuesassociated with the sensors receiving the subsequent flash whilecalculating the position of the one light flash.
 36. A method ofdetermining at least one of a position and an energy of an eventdetected by a medical imaging device, the method comprising:transforming each received radiation event into a light energy event;with an array of sensors, converting each light energy event into aplurality of output pulses; determining when a radiation event occursfrom the sensor output pulses; weighting each sensor output pulse as afunction of a distance between the respective sensor and the position ofthe event; and determining at least one of the position and the energyof the event from the weighted sensor output pulses.
 37. The method ofdetermining at least one of a position and an energy of an event as setforth in claim 36, wherein the step of determining at least one of theposition and the energy includes: calculating the energy E of the eventas: ${E = {\sum\limits_{i}{w_{i}^{E}S_{i}}}},$

where S_(i) represents the respective sensor output pulses, and w_(i)^(E) represents weighting values.
 38. The method of determining at leastone of a position and an energy of an event as set forth in claim 37,further including: determining an initial position x₀ of the event as afunction of the respective distances of the sensors from the position ofthe event.
 39. The method of determining at least one of a position andan energy of an event as set forth in claim 38, further including:determining a corrected position x of the event as:$x = \frac{\sum\limits_{i}{w_{i}^{x}S_{i}x_{i}}}{\sum\limits_{i}{w_{i}^{x}S_{i}}}$

where x_(i) represents respective sensor locations and where w_(i) ^(E)and w_(i) ^(x) represent weighting values that are a function of therespective distance |x_(i)−x₀| between the sensor location x_(i) and theinitial position x₀ of the event.
 40. The method of determining at leastone of a position and an energy of an event as set forth in claim 39,further including: determining the weighting values w_(i) ^(E) and w_(i)^(x) from an empirically generated optimum weighting graph.
 41. Themethod of determining at least one of a position and an energy of anevent as set forth in claim 37, further including: determining theweighting values w_(i) ^(E) and w_(i) ^(x) from pre-correction curves asa function of the respective distance |x_(i)−x₀| and as a function ofthe energy of the event.
 42. The method of determining at least one of aposition and an energy of an event as set forth in claim 41, furtherincluding: determining the weighting values w_(i) ^(E) and w_(i) ^(x) asa function of a scaling curve representing a relationship betweenvarious ones of the pre-correction curves.
 43. The method of determiningat least one of a position and an energy of an event as set forth inclaim 39, further including: determining the initial position x₀ of theevent as a centroid of the event.
 44. The method of determining at leastone of a position and an energy of an event as set forth in claim 36,wherein the step of determining the position of the event includes:ignoring any of the sensor output values of a sensor having an outputvalue that reaches a trigger amplitude after a delay period followingthe radiation event.
 45. The method of determining at least one of aposition and an energy of an event as set forth in claim 44, wherein thestep of determining the energy of the event includes: ignoring any ofthe sensor output values of a sensor having an output value that reachesthe baseline amplitude before the radiation event.
 46. A method forgenerating an image from a radiation event detected by a nuclear camera,the method comprising: detecting a plurality of output signals, each ofthe output signals being associated with one of a current radiationevent, a previous radiation event, and a subsequent radiation event;identifying when the current radiation event occurs as a function of theoutput signals; integrating the output signals associated with thecurrent radiation event; and generating the image as a function of theoutput signals integrated from the current radiation event.
 47. Themethod for generating an image from a radiation event as set forth inclaim 46, further comprising: determining if any of the output signalsare associated with the previous radiation event.
 48. The method forgenerating an image from a radiation event as set forth in claim 47,wherein the determining step includes: determining if any of the outputsignals exceed a predetermined value.
 49. The method for generating animage from a radiation event as set forth in claim 40, wherein theintegrating step includes: ignoring the output signals associated withthe previous radiation event.
 50. The method for generating an imagefrom a radiation event as set forth in claim 49, wherein the ignoringstep includes: reassigning the output signals associated with theprevious radiation event to be about zero.
 51. The method for generatingan image from a radiation event as set forth in claim 46, wherein theidentifying step includes: determining when one of the output signalssurpasses a trigger amplitude.
 52. The method for generating an imagefrom a radiation event as set forth in claim 51, further comprising:during the integrating step, determining if any of the output signals isassociated with a subsequent radiation event.
 53. The method forgenerating an image from a radiation event as set forth in claim 52,wherein the integrating step occurs during an integration period, thestep of determining if any of the output signals is associated with asubsequent radiation event including: determining if any of the outputsignals surpasses the trigger amplitude after a delay period beginningsubstantially simultaneously with the integration period.
 54. A methodof determining weighting values for correcting at least one of aninitially determined position and an initially determined energy of aradiation event, the method comprising: generating a plurality offall-off curves, each of the fall-off curves corresponding to arespective one of a plurality of energies; creating a plurality ofenergy ratio curves as a function of the fall-off curves, each of theenergy ratio curves representing a relationship between a selected pairsof the energies; determining a weighting value from one of the energyratio curves for scaling the fall-off curve associated with one of theenergies; and correcting the at least one of the initially determinedposition and the initially determined energy as a function of theweighting value and the fall-off curve associated with the initiallydetermined energy.
 55. The method of determining weighting values as setforth in claim 54 further including: generating an energy scaling curverepresenting a relationship between the energy ratio curves, thedetermining step also determining the weighting value as a function ofthe energy scaling curve.
 56. The method of determining weighting valuesas set forth in claim 54, wherein the step of generating each of thefall-off curves includes: dividing a selected fall-off curve by anactual fall-off curve, each of the fall-off curves representing anenergy amplitude as a function of a distance.
 57. The method ofdetermining weighting values as set forth in claim 54, furtherincluding: before the creating step, normalizing the fall-off curves.